As far back as the early 1990s, the electrical power industry was looking for ways to standardise the increasing numbers of ‘intelligent electrical devices’ used in substation equipment. Up to that point, much of the interconnection was hardwired and intelligent communication was commonly achieved with serial protocols. These efforts eventually led to a new IEC 61850 standard titled ‘Communication networks and systems for power utility automation’. The standard was needed to ensure real-time substation automation applications could be implemented using digital communication networks.
Driving towards a standard
This standard was initially released in the early 2000s, and it defined communications methods and services to promote high inter-operability among all makes of hardware. More recently, this standard has been updated to what is commonly known as IEC 61850 Edition 2 to add extensions and improvements, and to correct various shortcomings.
Key portions of IEC 61850 identify data structures, commands and conformance testing requirements. Without rigorous testing methodology and certification, it is impossible to ensure various products will inter-operate properly and meet the constraints defined within the standard. Therefore, it is imperative that any devices to be integrated within a network are in compliance with the IEC 61850 standard, which means that each device must undergo conformance testing by a suitable agency and receive a certificate.
One update in IEC 61850 Edition 2, from the previous version, was the addition of maximum allowable recovery time requirements for various communication events, which can only be achieved by means of higher availability for communications links. Fortunately, the rise in prominence of industrial Ethernet has provided various redundancy solutions meeting these needs. As Electric Light & Power states, “With Ethernet advancements, communication is no longer a limiting factor inside or outside substations.” Furthermore, it states that the engineering definition and structure defined in the standard will simplify engineering and integration for the technical team.
IEC 61850 is able to fulfill the link redundancy requirements in part by referencing IEC 62439-3, which is titled ‘Industrial communication networks - High availability automation networks - Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR)’. It identifies methods, protocols, and topologies for achieving Ethernet network redundancy.
Benefits of hardware-based network redundancy
Clearly, the communication redundancy requirements indicated by the IEC standards set the direction for the power utility automation industry. Solutions must be compliant with the IEC standards. Specific approaches employing hardware-based network redundancy solutions in compliance with IEC standards offer a number of benefits. These include being compliant with IEC 61850 Edition 2 to provide a total communication redundancy solution, promoting interoperability, mixed topologies possible, being able to connect with non-redundant networks, not impacting CPU loading and simplified network upgrades.
Perhaps the most compelling reason to select hardware-based redundancy is that all elements will unmistakably be certified as compliant with a common standard, leading to a total solution. Any other hybrid approach using non-compliant devices or a mix of hardware and software calls into question whether it is truly compliant with the standards. In direct contrast to the old way of doing things, HSR and PRP are specified to be interoperable with each other, and among devices complying with these standards. It is most straightforward to implement HSR as a ring and PRP as a parallel star. However, there are times when the architecture may require more complex or mixed topologies.
Hardware-based components are available to handle these situations in a standard manner. In fact, specific devices called redundancy boxes (RedBoxes) are available to allow any one-port device or non-redundant network to connect seamlessly into a redundant HSR or PRP network.
Looking at performance, hardware-based network redundancy devices are purpose-built for this role. Therefore, they handle all redundancy functions on board and are completely transparent to any external devices. Not only does this simplify implementation, but it means that there is no additional CPU loading for any other devices. In the case of industrial automation computers, embedding all the redundant networking functionality on the network adapter ensures the computer can operate optimally and not be hindered in any way by networking issues.
Power automation network redundancy requirements
What exactly does it mean for hardware to offer the right redundancy capabilities for switchgear and substation networking? The IEC standards spell out the performance requirements and indicate some ways of achieving them. They include being IEC 61850 certified, achieving mandated recovery times, ensuring zero data loss, making ring topology and redundant star topology possible.
To start with, hardware vendors must submit their products to testing agencies for evaluation against the IEC 61850 standard in a form of simulated service to achieve certification. Without this certification, the product should not be considered for substation automation applications. Typically, devices are tested individually to confirm basic functionality, and also in conjunction with other related devices to confirm interoperability. Networking traffic will also be evaluated. Common Ethernet protocols in the IT world are neither deterministic nor do they guarantee that a packet will ever actually make it to the destination. There are some other more advanced protocols in the IT world to address recovery time and redundancy, such as, Rapid Spanning Tree Protocol (RSTP), but they are nowhere near fast enough for the most demanding IEC 61850 requirements.
So, how is it that common Ethernet media can be considered and leveraged for substation automation? As noted earlier, IEC 61850, in turn, refers to IEC 62439-3 to introduce high-performance redundancy protocols acceptable for meeting the recovery time requirements. HSR and PRP are specifically defined methods of achieving suitable redundancy, capable of meeting the zero-recovery time requirement since their architecture ensures no packet is lost. They share some similarities but also have their own pros and cons.
HSR network uses a ring topology and requires no dedicated switches. Instead, each intelligent device has at least two ports and acts as a switch so each data packet frame received on a given port is retransmitted out the other port. The basic concept is that if the ring is healthy, each destination node should receive two identical frames from a source node, with minimal time delay between the two. Normally, the second frame is discarded, but if it is never received, that indicates trouble on one of the paths. Even in the case of one break in the ring, operation continues normally.
The PRP approach is a parallel star. Every network path consists of two connections, effectively creating two networks completely in parallel. Every frame is sent from each source to each destination down both paths. Again, the destination device normally receives both frames and discards the second, but any time only one frame is received, it indicates trouble on the other path. As with HSR, the operation continues normally even in the event of a failure.
As communication technology continues to focus on Ethernet, and as the electrical power industry complies with IEC 61850, applications will naturally standardise to implementing HSR and PRP. HSR has some limitations on node quantity and bandwidth, and requires specialised node hardware but avoids the need for additional switches. PRP allows more common network methods, but doubles the number of switches and field cables. Each installation must be evaluated to identify if one or both solutions are optimal.
Adopting standardised technologies
Critical energy infrastructure, such as, electrical substations and switchgear used for power transmission and distribution, serve a demanding role in society. It is crucial that automation methods applied to these systems offer the highest level of performance. In particular, many of the intelligent electrical devices used in energy infrastructure systems offer advanced communications capabilities over and above their basic protective features. Furthermore, industrial Ethernet has made great advances in all types of automation. However, more is involved than simply patching switchgear devices into an Ethernet switch.
Since the early 2000s, IEC 61850 and 62439 standards have developed to identify the required means for ensuring reliable power utility automation using networked devices. Some of the most critical communication events are required to experience zero data loss. The standards define two network topologies that can readily achieve this requirement. HSR rings are economical to implement, while PRP parallel networks require more switches and media but offer better performance. When implemented properly, even high speed protection and control can be performed over Ethernet, which reduces installation complexity.
The industry is moving to adopt these standardised technologies, and the number of installed applications will continue to increase. Users must select products that have been tested and certified to these standards for their power utility automation applications. Hardware-based products are available to provide a complete solution for many different topologies, without negatively impacting intelligent device performance. For instance, industrial automation leader, Advantech, offers the ECU-4784 power automation computer and the ECU-P1524PE SFP gigabit Ethernet adapter card, both of which are certified as IEC 61850 compliant, allowing users to integrate significant computing and networking capabilities into their switchgear automation projects.
Hardware-based network redundancy standards and products are available and offer the preferred method for achieving electrical utility automation. Attempting to implement the required levels of performance, reliability and interoperability with non-compliant hardware or with software-based solutions will result in an inferior solution and should be avoided. When correctly specified and implemented, these hardware-based network redundancy standards and products will provide years of trouble-free service, along with the ability to easily and quickly perform upgrades.
Courtesy: Advantech Industrial Computing India